Basement tectonic structure and sediment thickness of a valley defined using HVSR geophysical investigation, Azuela valley, Ecuador

Abstract

[EN] Using a small set of boreholes, as fixed information of basement, combined with the analysis of microtremor surveys, can provide a transversal detailed section of a valley where a dam was constructed. In this paper, horizontal to vertical spectra ratio (HVSR) technique is applied as a quick and economic method to establish the thickness of the sediments existing over rock basement in the San Marcos dam area, constructed in the Azuela valley (Cayambe, Ecuador). Previous investigations made for construction of dam did not reach the bottom of the 700 m transverse section of dam foundation; only the ends were well deferred, where the rock is close to surface. So, involving some little number of boreholes as control points, in comparison with other similar studies, a relation between natural frequency of ground vibration (fo) and sediment thickness is established. Along the dam toe, and over it, was measured 20 HVSR single station points and analysed in the three main directions (N-S, E-W, and Z) the components of ground natural vibrations (rumour). This provides the variation of the ratio between the horizontal components and the vertical one and using the natural frequency of vibration in each point can be established a correlation between this one and sediment thickness. After this, the geological cross-section of the bottom of the valley was delineated, revealing tectonic structures (faults) not defined before that they can be useful in further studies for control of settlement of dam. The new formula could be used too as a quick tool to investigate accurately the area around the dam and defined other tectonic structures not evidenced at this moment.